1. Field of the Invention
The present invention is directed to a process for the removal of hydrogen sulfide from supercritical carbon dioxide (CO.sub.2).
2. Description of the State of the Art
Supercritical (fluid) carbon dioxide (CO.sub.2) is different from conventional hydrocarbon or by-product liquids and gases in that it is a dense fluid which is compressible like a gas but unlike a gas is pumpable like a liquid. It is desirable to pipeline CO.sub.2 as a supercritical single phase fluid, for ultimate use, e.g., as an injection fluid in secondary and tertiary oil recovery, and to have a sweet (essentially H.sub.2 S-free) fluid to transport and utilize as an injection fluid.
Generally CO.sub.2 produced from wells is depressurized, purified in the gaseous state, and repressurized at the field location for pipelining in its supercritical state. This requires (1) sizable on-site equipment for depressurizing and repressurizing the CO.sub.2, solvent handling and (2) high energy demands. In the case of supercritical CO.sub.2, most of the conventional sorbents used to remove H.sub.2 S selectively from gases such as natural gas, for example, aqueous methyldiethanolamine or other amines in aqueous solution, or physical solvents such as Selexol, or mixed solvents such as Sulfinol, are not useful because (1) the partial pressure of CO.sub.2 is so high that solvent selectivity is impaired, or (2) the solvent and the supercritical CO.sub.2 do not form two phases--an essential condition for any form of solvent treatment. Moreover, supercritical CO.sub.2 is a very good solvent, and undesirably dissolves many potential treating solvents such as these.
U.S. Pat. No. 4,545,965 describes a process for the purification of gases, e.g., natural gas, containing H.sub.2 S in the presence of minor amounts of CO.sub.2 using a solution of a tertiary amine and a variety of selective solvents, including sulfolane, N-methylpyrrolidone, glycol (ethers), amines and certain heterocyclic-nitrogen compounds. Water is an undesirable ingredient, which may be "tolerated" in limited amounts, such as less than 2%. Indeed, the presence of water in Examples 7 and 8 of U.S. Pat. No. 4,545,965 increased the amount of CO.sub.2 sorbed over that in the absence of water in Example 1. Also, applicant has found that the sulfolane co-sorbent of Examples 1 and 5 through 8 of the patent and many other co-sorbents of the patent are not effective co-sorbents for substantially supercritical CO.sub.2 because of the unique properties and requirements of sorbents for substantially supercritical CO.sub.2 as compared to the requirements for sorbents for conventional hydrobcarbon or by-product liquids and gases, e.g., natural gas, coal gasification products and the like.
Suprisingly, it has been found that certain solvent systems are good solvent systems for removing H.sub.2 S from supercritical CO.sub.2, thus avoiding the problems of depressurization and repressurization and high energy demands while handling the unique properties of supercritical CO.sub.2 by being a much better solvent for H.sub.2 S than supercritical CO.sub.2 is for the solvent.